The present invention relates to the art of medical imaging. It finds particular application in conjunction with C-arm supports for generating three-dimensional computed tomography imaging data, more particularly fluoroscopic x-ray systems, and will be described with particular reference thereto. It is to be appreciated, however, that the invention is also applicable to other real-time imaging systems capable of monitoring a region of a patient during a minimally invasive procedure.
In some operating rooms, such as operating rooms for vascular catheter procedures, a projection x-ray imaging device is provided in association with the operating table. More specifically, an x-ray tube or generator and an x-ray detector are mounted on a C-arm which is positioned such that the area of interest or patient lies between the x-ray source and detector. The x-ray source and detector are rotatable and longitudinally displaceable as a unit to select a region and angle for projection imaging. Once the surgeon has positioned the x-ray source and detector in the proper position, the surgeon actuates the x-ray tube sending x-rays through the patient and onto the x-ray detector for a preselected exposure time. The x-rays received by the detector are converted into electronic, video image data representing a projection or shadow-graphic image. The projection or shadow-graphic image is displayed on a video monitor which is viewable by the physician.
In cardiac catheterization procedures, for example, images are generated to show the vasculature system and monitor the advance of the catheter through the blood vessels. More specifically, the surgeon advances the catheter into the patient, stops the surgical procedure, and initiates an x-ray imaging procedure. The x-rays are converted into electronic data and a projection image is displayed.
One of the drawbacks of these x-ray systems is that the resultant image is a projection or shadow-graphic image. That is, the 3-D vasculature system of the patient is projected into a single plane.
If 3-D diagnostic images are required, such images are often taken with a CT scanner or a magnetic resonance imaging device which is typically located in another part of the facility. Thus, any three-dimensional diagnostic images are commonly generated sometime before the surgical procedure starts. Even if a CT scanner is present in the surgical suite, the patient is still moved into the scanner. The transportation of the patient to the CT or MRI machine for further imaging often renders three-dimensional images impractical during surgery.
However, three-dimensional images obtained are valuable during surgical procedures. After generating a three-dimensional diagnostic image, a surgical procedure is commenced, such as a biopsy. From time to time during the procedure, additional projection diagnostic images are generated to monitor the advancement of the biopsy needle into the patient. The location of the needle can be mathematically predicted from the projection images and monitoring of the physical position of the needle or other instrument can be superimposed on the 3-D diagnostic images. As the needle moves, the superimposed images can be altered electronically to display the needle in the proper position. Various trajectory planning packages have been proposed which would enable the operator to plan the biopsy procedure in advance and electronically try various surgical paths through the three-dimensional electronic data.
Recently, there has been some interest in using relatively low power fluoroscopic systems to generate real time three-dimensional CT reconstructions. Such a technique, disclosed in U.S. Ser. No. 08/802,618 to Barni, is assigned to the assignee of this invention. Barni suggests operating the x-ray tube of a CT scanner in a fluoroscopic mode. Unfortunately, the complete, encircling CT gantry can obstruct access to the surgical site or make that access inconvenient or uncomfortable for the physician.
Another solution disclosed by R. Fahrig, et al. in SPIE Volume 2708 entitled "Characterization Of A C-Arm Mounted XRII For 3-D Image Reconstruction During Interventional Neuro Radiology" recognizes that a C-arm would provide improved access to the surgical site. The Fahrig article also observes that the C-arm lacks sufficient rigidity to prevent the x-ray source and the detector plates from moving relative to each other, especially during a volume scan where the source and the detector are rotated about an area of interest. Relative motion misaligns the apparatus and causes image degradations. The Fahrig article describes a method wherein the motions and the deflections of the C-arm are premeasured or estimated in pilot scans. The deflections are assumed to remain the same for subsequent scans performed from the same starting point and within all other parameters. System calibration is performed by inserting a three-dimensional phantom containing metal beads or the like with known locations into the imaging field and performing a representative scan. Subsequent image analysis is used to determine positional errors, due to C-arm distortion and deflection. By comparing the detected position of the beads in each image with calculated ideal positions that would occur in the absence of any C-arm distortion errors are calculated. These errors, for each image scan, are stored in a long-term memory and applied to all subsequent scans, correcting for the calibration errors. Unfortunately, the Fahrig method requires that all volume imaging scans begin in exactly the same location and travel through the same arc. Moreover, any changes in the mechanical characteristics of the C-arm, such as bearing wear, changes in the source to image distance, drive speed, etc., will cause a deterioration in image quality due to the application of improper positional corrections.
The present invention provides a new and improved method and apparatus which overcomes the above-referenced problems and others.